In recent years, from the viewpoint of global environmental conservation, an improvement in fuel efficiency in automobiles has been an important issue. To address this issue, there is a strong movement under way to strengthen body materials to decrease the thickness of components, thereby decreasing the weight of bodies. However, an increase in strength of steel sheets causes a decrease in ductility, resulting in poor formability. Thus, under the existing circumstances, there is a demand for the development of high-strength materials with improved formability.
Furthermore, taking into account a recent growing demand for high corrosion resistance of automobiles, galvanized high-strength steel sheets have been developed frequently.
To satisfy these demands, various multiphase high-strength galvanized steel sheets, such as ferrite-martensite dual-phase steel (DP steel) and TRIP steel, which utilizes the transformation-induced plasticity of retained austenite, have been developed.
For example, JP 11-279691 proposes a high-strength galvannealed steel sheet with excellent formability that includes C: 0.05% to 0.15%, Si: 0.3% to 1.5%, Mn: 1.5% to 2.8%, P: 0.03% or less, S: 0.02% or less, Al: 0.005% to 0.5%, and N: 0.0060% or less, on the basis of mass percent, and Fe and incidental impurities as the remainder, wherein (Mn %)/(C %) is at least 15 and (Si %)/(C %) is at least 4. The galvannealed steel sheet contains 3% to 20% by volume of martensite phase and retained austenite phase in a ferrite phase. Thus, in a technique disclosed by JP 11-279691, a galvannealed steel sheet with excellent formability contains a large amount of Si to maintain residual γ, achieving high ductility.
However, although DP steel and TRIP steel have high ductility, they have poor stretch flangeability. The stretch flangeability is a measure of formability in expanding a machined hole to form a flange. The stretch flangeability, as well as ductility, is an important property for high-strength steel sheets.
JP 6-93340 discloses a method for manufacturing a galvanized steel sheet with excellent stretch flangeability, in which martensite produced by intensive cooling to an Ms point or lower between annealing/soaking and a hot-dip galvanizing bath is reheated to produce tempered martensite, thereby improving the stretch flangeability. However, although the stretch flangeability is improved by the transition from martensite to tempered martensite, EL is low.
As a high-tensile galvanized steel sheet with excellent deep drawability and stretch flangeability, JP 2004-2409 discloses a technique in which C, V, and Nb contents and annealing temperature are controlled to decrease the dissolved C content before recrystallization annealing, developing {111} recrystallization texture to achieve a high r-value, dissolving V and Nb carbides in annealing to concentrate C in austenite, thereby producing a martensite phase in a subsequent cooling process. However, this high-tensile galvanized steel sheet has a tensile strength of about 600 MPa and a balance between tensile strength and elongation (TS×EL) of about 19000 MPa·%. Thus, the strength and ductility are not sufficient.
As described above, the galvanized steel sheets described in JP 11-279691, JP 6-93340 and JP 2004-2409 are not high-strength galvanized steel sheets with excellent ductility and stretch flangeability.
In view of the situations described above, it could be helpful to provide a high-strength galvanized steel sheet that has a TS of at least 590 MPa and excellent ductility and stretch flangeability and a method for manufacturing the high-strength galvanized steel sheet.